1. Field of the Invention
The invention relates to a drawing device in a press for drawing shaped sheet-metal parts and, more particularly, providing variable and reproducible support of the sheet-metal holder as may be required during a drawing operation.
2. Description of the Related Art
Known hydraulic drawing devices for presses for drawing shaped sheet-metal parts generally consist of a top, vertically movable drawing ram as a top tool part and a bottom drawing punch, resting in a stationary position on a press table, as a bottom tool part, between which the shaped sheet-metal part to be formed is inserted. In order to avoid cracking in the wall or folds or corrugations in the surface structure of the workpiece, in particular in the case of workpieces of large area or also in the case of parts of complicated shape, this workpiece is held during the actual drawing operation by a drawing frame, holding-down device or sheet holder which surrounds the drawing punch in one piece or in several pieces. As apparent, for example, from EP 0,074,421, such a sheet holder is supported from below via a number of pressure bolts which are in turn supported relative to a vertically movable pressure plate, drawing cushion or pressure cheek. To this end, it has become known from DE 3,242,705 C2 from the applicant to regulate the pressure cheek in its vertical adjustment in its corner areas via a plurality of pressure-cylinder units, which can be regulated separately from one another, in order to apply a varying degree of force to the pressure bolts lying above them. In this way, the pressure cheek can be charged during the entire drawing process with locally varying, desired forces in any desired fine adjustment steps, with the pressure cheek further transmitting these forces by way of the pressure bolts to the sheet holder so as to adapt its contact pressure to the existing requirements.
The last-mentioned arrangement certainly permits good and partial support of the pressure cheek so that tilting can largely be avoided in the event of eccentric loading of the pressure cheek. However, the use of bolts of more or less the same length, which are inserted in a number corresponding to the requirement for the application of force to the sheet holder, has the disadvantage that these bolts can "dig" into both the sheet holder and the pressure cheek and may possibly additionally be compressed themselves. Reproducible relationships during refitting of the tool cannot as a rule be achieved in this way.
In order to avoid the use of insertable pressure bolts, it was therefore also proposed that the sheet holder be directly supported by a plurality of cylinder units arranged below it, which cylinder units act directly on the sheet holder by means of a separate pressure-displacement control or regulating system. In this arrangement, generally four cylinders were used which symmetrically support the sheet frame. Such an arrangement certainly has the advantage that accurate control or regulation of the application of force to the sheet holder is possible. However, the small number of cylinder units supporting the sheet holder has the disadvantage that the support itself is inadequate in certain areas and in the case of parts of complicated construction. Instead of four cylinders, a multiplicity of cylinders could certainly be arranged below the sheet holder in the press table. However, a spatial limit occurs here, since each cylinder has to be provided with a proportional valve control system so that a separate pressure-displacement control or regulating system can be realized. Furthermore, such a method would probably be very cost-intensive.
Furthermore, EP 0,173,755 A1 has disclosed a drawing device for a press according to the generic category of the present invention, in which drawing device a row of additional pressure-cylinder units is provided between pressure cheek and sheet holder. The pressure cheek itself is controlled in its vertical position by at least one pressure unit located below it. Both the individual pressure cylinders between pressure cheek and sheet holder and the pressure unit for the pressure cheek itself can be actively or passively subjected to a control or regulating system.
In this known device, the application of pressure to the sheet holder and thus the counterforce, acting from below, of the sheet holder against the drawing ram are effected essentially by the pressure-cylinder units arranged between sheet holder and pressure cheek. These pressure-cylinder units are in each case designed as stationary pressure cylinders having a continuous piston rod projecting on either side, the bottom part of the piston rod acting on the pressure cheek and the top part of the piston rod acting on the sheet holder. Passive regulation, generally provided, of the outflow of the pressure medium produces the desired counterholding force on the sheet holder, the counterholding force between sheet holder and top tool part during the drawing operation being produced solely by the pressure cylinders, arranged above the pressure cheek, upon appropriate activation. The pressure cheek of the known device, which pressure cheek can be acted upon centrally by pressure from below, performs a sequence of movements synchronously following the ram movement and serves on the one hand to raise the sheet holder into the position for inserting the workpiece and on the other hand to eject the workpiece into the transport plane. Locally varying support of this pressure cheek and application of force to the same is not provided during the drawing operation.